1. Field of the Invention
The present invention relates to an imaging optical system, and an image pickup apparatus using the same, and particularly to an image pickup apparatus in which a solid image pickup element such as a CCD (Charge Coupled Device) and a CMOS (Complementary Metal Oxide Semiconductor) is used.
2. Description of the Related Art
As an image pickup apparatus in which an image pickup element (e.g., CCD and CMOS) and an imaging optical system are used, a car-mounted camera is known. Moreover, in the recent years, automobiles provided with a car-mounted camera are becoming popular. The conventional car-mounted cameras were being used mainly as a visual monitor. Recently, the car-mounted camera has been used as a camera for a higher level sensor, such as a sensor for a white line detection and monitoring of traffic line deviation, an obstacle detection, and driver monitoring. An imaging optical system used in these image pickup apparatuses is sought to be an imaging optical system having a small F number to enable imaging of a favorable quality image even at night, and which is bright and capable of securing an image angle.
As examples of an imaging optical system in which number of lenses forming the lens system is small, the image angle is wide, and the F number is comparatively bright, and in which a back focus and a telecentricity suitable for an image pickup apparatus is secured, an imaging optical system described in Japanese Patent Application Laid-open Publication No. 2003-307671, and an imaging optical system described in Japanese Patent No. 3756114 are known.
The optical system described in Japanese Patent Application Laid-open Publication No. 2003-307671 has a half-image angle of 30° which is comparatively wide, and the F number of 2.8. Moreover, the optical system described in Japanese Patent No. 3756114 has the half-image angle of 30° which is comparatively wide, and the F number of about 3.5.